Gearbox issue on F4, engine swap?
 

Hello all! I'm new to the forum, just bought a CBR but I have issues with it :icon_eek:

I bought it last month, a 1999 F4 with only 20k miles on it, some paint work on it. It worked great except from time to time losing 2nd gear when upshifting without clutch.

Just last weekend on an accelleration it was hesitating between 2nd and 3rd by itself, then I heard a big clacking noise and lost all power, on all gears except 5th. On all gears even neutral, i get a sound at each rotation where the gear seems to slip, with some kind of friction. On neutral the same thing happens, it moves the rear wheel a little bit also. I've made a video i'll try to upload it.

Strangely the 5th gear is fine. So I suspect there's something wrong in the gears or selection forks. I was thinking of doing the work myself but it seems like a massive task to open the crankcase. There's not many tutorials either since it seems that honda gearboxes never fail!

Now I was also thinking of swapping another engine, but it seems the F4 is fairly rare, it was built just for a year here (France).

There are plenty of F4i engines available, and also F3 engines. Here the Hornet 600 was a big hit so I can find a hornet engine PC25, which is a F3 engine, for like 300$, instead of 1000$ for a f4.

So the question is, can I swap a f3 or f4i on my f4 with minimal adaptation? or should I be patient and look for a f4 engine? or is it not that hard to open the gearbox?

Thanks!

https://cimg6.ibsrv.net/gimg/cbrforu...b8341f30ef.jpg

F3: No, not a chance.
F4I: Maybe....

well at least I found the reason for my problems, there are some gears parts in the oil pan.

Should I look for damage somewhere else than on the gearbox?



https://cimg9.ibsrv.net/gimg/cbrforu...ecce0d75fd.jpg

It took me a couple of month to open it but here it is. There is damage on the gears.

The metal shards seem to have gone through the oil pump, there are dents in it, I assume it needs to be changed.

Would a swap with a complete gearbox fix it or is there anything else to look at?

Thanks
https://cimg4.ibsrv.net/gimg/cbrforu...ea0313e46e.jpg
https://cimg7.ibsrv.net/gimg/cbrforu...9642085441.jpg
https://cimg0.ibsrv.net/gimg/cbrforu...97dab87c6e.jpg

I'm going through a similar thing because the dogs for 2nd gear wore to the point that it pops out into neutral.
The teeth are usually pretty strong compared to the dogs on sliding mesh boxes, but if a shift fork is bent and it tries to go into two gears at the same time, this could happen.

Are your shift forks bent/broken/worn?

I don't know about your specific gears, but the two that I needed are NLA, so I ordered a full F4i set of gears to see if they fit. They probably show up in a week.
I can confirm that 600RR gears are completely different though.

I looked at the dogs on the gearbox, I don't know really what to look for but they do not seem bad to me, a little worn but nothing bad.

While disassembling though I noticed one of the forks was out of tolerance, and has marks on it.

I ordered a full F4 gearbox to replace it. To my surprise the same fork is even more damaged on the gearbox I received, so I'll keep the two good ones.

F4 swingarm pivots off of the back of the motor so simplest swap would be another F4 or go F4i and you'll need harness, ecu and fuel tank.

Were you able to verify if the complete F4I gearbox fits in the F4? The dogs of the 3/4 gear on the new gearbox I received are actually worn, and the replacement is not produced anymore.

It does not. The F4i has different spacing between main and countershaft, so they weren't a direct swap for me.
Some of the gears are the same (the large 33T gear I mentioned in my thread) ended up being the same. So I guess the NLA F4 gear 23441-MBW-000 can be directly replaced with the still-available F4i gear 23441-MBW-J20.

For the other gear (23521-MBW-000), I brought it to a machine shop and had them re-face the dogs. All they should need is a dividing head (practically guaranteed that any machine shop will have one) and warning that it's a hardened face.
The 2nd gear dogs are already exceptionally thick and people have done this before to "back cut" the dogs, so there's no worry about remaining strength. I had it refaced to be flat like stock so I don't need to machine the opposite gear as well.
The corner that originally rounded off was hardened before, but that hardness has some very small thickness from the outer face of the dog so the very first part of the surface to make contact shouldn't be too soft.

You could potentially try something similar by machining those gears, or take a look at the IPC (I've had good luck with this one https://www.hondapartshouse.com/oemp...torcycle/parts because others seem to remove parts that they don't sell...) for both bikes and take a gamble if the teeth numbers are the same for F4 and F4i.